June 14, 2007
01:00 PM (EDT)

News Release Number: STScI-2007-24

Astronomers Measure Mass of Largest Dwarf Planet

The full news release story:

NASA's Hubble Space Telescope has teamed up with the W.M. Keck Observatory to
precisely measure the mass of Eris, the largest member of a new class of dwarf
planets in our solar system. Eris is 1.27 times the mass of Pluto, formerly the
largest member of the Kuiper Belt of icy objects beyond Neptune.

Hubble observations in 2006 showed that Eris is slightly physically larger than
Pluto. But the mass could only be calculated by observing the orbital motion of
the moon Dysnomia around Eris. Multiple images of Dysnomia's movement
along its orbit were taken by Hubble and Keck.

Astronomer Mike Brown of the California Institute of Technology in Pasadena,
Calif. and colleagues also report in this week's Science Magazine that Dysnomia
is in a nearly circular 16-day orbit. This favors the idea that Dysnomia was born
out of a collision between Eris and another Kuiper Belt object (KBO). A
gravitationally captured object would be expected to be in a more elliptical orbit.

The satellites of Pluto, as well as the Earth-Moon system are also believed to
have been born out of a collision process where debris from the smashup goes
into orbit and coalesces into a satellite.

By comparing the mass and diameter, Brown has calculated a density for Eris of
2.3 grams per cubic centimeter. This is very similar to the density of Pluto, the
large Kuiper Belt object 2003 EL61, and Neptune's moon Triton which is likely a
captured KBO. These higher densities imply that these bodies are not pure ice
but must have a significant rocky composition.

The discovery of Eris in 2005 (originally nicknamed Xena, and officially cataloged
2003 UB313) prompted a debate over the planetary status of Pluto because
astronomers realized they would have to call it the "10th" planet if Pluto retained
its own planetary status, which was already under debate. This led the
International Astronomical Union, in 2006, to make a new class of solar system
object called dwarf planets. These are spherical bodies in hydrostatic equilibrium
(objects that have sufficient gravity to overcome their own rigidity and form a
spherical shape) like the planets, but unlike the major planets in the solar system,
they have not gravitationally cleared out the neighborhood of particles and small
debris along their orbits.